1. Field of the Invention
The present invention relates to imaging members and to the preparation of a structurally simplified imaging member which does not exhibit curling of the multilayered imaging member webstock after coating and drying of the charge transport layer.
An advantage of the present invention is to provide improved methodology for fabricating multiple layered imaging member webstocks which overcomes curling of the multiple layers.
The present invention provides an improved process for imaging member webstock fabrication having a simplified material configuration.
2. Description of Related Art
Electrostatographic flexible imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example, (1) photosensitive members (photoreceptors) commonly utilized in electrophotographic processes and (2) electroreceptors such as ionographic imaging members for electrographic imaging systems. The flexible electrostatographic imaging members may be seamless or seamed belts. Electrophotographic imaging member belts comprise a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl backing layer coated on the opposite side of the substrate layer. Some electrographic imaging member belts have a more simple material structure comprising a dielectric imaging layer on one side of a supporting substrate and an anticurl backing layer on the opposite side of the substrate.
Electrophotographic flexible imaging members may comprise a photoconductive layer comprising a single layer or composite layers. Typical electrophotographic imaging members exhibit undesirable imaging member curling and require an anticurl backing layer. The anticurl backing layer is provided to prevent the multiple layers of an imaging member from curling and thereby keeping the member flat. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990, which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. The supporting electrode may still function as an anode when the charge transport layer is sandwiched between the supporting electrode and the photoconductive layer. The charge transport layer in this latter embodiment is capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer. Photosensitive members having at least two electrically operative layers, as discussed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting image is transferable to a receiving member such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For flexible electrophotographic imaging members having a belt configuration, the numerous layers found in modern photoconductive imaging members are highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in negatively charging electrophotographic imaging systems consists of a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor belt may also comprise an additional layer such as an anticurl backing layer to achieve the desired imaging member belt flatness.
In a service environment, a flexible imaging member belt, mounted on a belt supporting module, is exposed to repetitive electrophotographic image cycling which subjects the outer-most charge transport layer to mechanical fatigue as the imaging member belt bends and flexes over the belt drive roller and all other belt module support rollers, as well as sliding bend contact above each backer bar""s curving surface. This repetitive imaging member belt cycling leads to a gradual deterioration in the physical and mechanical integrity of the exposed outer charge transport layer leading to premature onset of fatigue charge transport layer cracking. The cracks developed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print out defects which thereby adversely affect the image quality on the receiving paper. In essence, the appearance of charge transport cracking cuts short the imaging member belt""s intended functional life.
When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the charge transport layer coating and drying process, curling of the multilayered photoreceptor is observed and requires an anticurl backing layer applied to the backside of the substrate support, opposite to the side having the charge transport layer, to offset the curl and render the photoreceptor web stock flat. The exhibition of photoreceptor curling after completion of charge transport layer coating has been determined to be the consequent of thermal contraction mismatch between the applied charge transport layer and the substrate support under the conditions of elevated temperature, heating and drying the wet coating and the eventual cooling down to room temperature. Since the charge transport layer in a typical prior art photoreceptor device has a coefficient of thermal contraction approximately 3xc2xd times larger than the substrate support, the charge transport layer, upon cooling down to room ambient, results in greater dimensional contraction than that of the substrate support causing photoreceptor curling.
Although it has been desirable to have the anticurl backing layer to complete a photoreceptor web stock material package, an anticurl backing layer application represents an additional coating step increasing labor and material cost, which can result in a decrease of daily photoreceptor production through-put of about 25%. Moreover, sending the photoreceptor web stock back to the coater immediately after coating the charge transport layer for anticurl backing layer application has frequently resulted in photoreceptor production yield lost due to web stock scratching caused by handling. Photoreceptors with an anticurl backing layer have a built-in internal strain of about 0.28% in the charge transport layer. This strain is cumulatively added to each photoreceptor bending induced strain as the photoreceptor belt flexes over a variety of belt module support rollers during cycling within a machine. This internal built-in strain exacerbates the fatigue charge transport layer failure and promotes the onset of charge transport layer cracking.
Imaging members having an anticurl backing layer not only require one addition coating step to complete the finish production, but also create an environmental issue involving solvent emission release to the atmoshere.
Seamed flexible photoreceptor belts are fabricated from sheets cut from a electrophotographic imaging member web stock having anticurl backing layer. The cut sheets are generally rectangular in shape. All edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheet is formed into a belt by joining the overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping opposite marginal end regions at the point of joining. Joining may be effected by means such as welding (including ultrasonic processes), gluing, taping, or pressure/heat fusing. However, ultrasonic seam welding is generally the preferred method of joining because it is rapid, clean (no application of solvents) and produces a thin and narrow seam. The ultrasonic seam welding process involves a mechanical pounding action of a welding horn which generate a sufficient amount of heat energy at the contiguous overlapping marginal end regions of the imaging member sheet to maximize melting of one or more layers therein. A typical ultrasonic welding process is carried out by pressing down the overlapping ends of the flexible imaging member sheet onto a flat anvil and guiding the flat end of the ultrasonic vibrating horn transversely across the width of the sheet and directly over the overlapped junction to form a welded seam having two adjacent seam splashings consisting of the molten mass of the imaging member layers ejected to either side of the welded overlapped seam. These seam splashings of the ejected molten mass comprise about 40% by weight of the anticurl backing layer material.
In a related photoreceptor device, an anticurl backing layer having filler reinforcement for robust mechanical function may also have bubbles in the material matrix which negate and diminish the benefit of wear resistance enhancements, otherwise achievable through dispersion of inorganic or organic particles in the layer for increasing wear resistance. Also, due to the presence of bubbles, a weakening of the layer and onset of mechanical failure can occur when fatigue tension/compression strain is repeatedly applied to the anticurl backing layer during machine cycling, particularly when cycling around small diameter support rollers. Further, when rear erase is employed to discharge the photoreceptor belt during electrophotographic imaging processes, the presence of bubbles causes a light scattering effect which leads to undesirable non-uniform discharge. Also, the presence of bubbles in the anticurl backing layer during seam welding processes can cause the bubbles to expand and form splashings exhibiting open pits. During electrophotographic imaging and cleaning cycles, these open pits can function as sites that trap toner, debris, and dirt particles making attempts to clean the imaging member belt extremely difficult. It has also been found that, during imaging belt cycling, the trapped toner, debris, and dirt particles can be carried out by the cleaning blade from the pits to contaminate the vital imaging components such as the lenses, Hybrid Scavengeless Development subsystems (HSD), Hybrid Jumping Development subsystems (HJD) and, other subsystems, and can also lead to undesirable artifacts which form undesirable printout defects in the final image copies.
Another disadvantage of photoreceptors having an anticurl backing layer occurs under dynamic belt cycling function conditions. The anticurl backing layer is in constant mechanical interaction with the machine belt support rollers and backer bars causing the anticurl backing layer to develop substantial premature wear problems. Anticurl backing layer wear reduces the thickness of the anticurl layer and diminishes the desired flattening effect. This loss of anticurl layer thickness results in non-uniform charging density at the photoreceptor belt surface under normal imaging processing conditions.
With the above noted undesirables mentioned, fabrication of flexible seamed photoreceptor belts without the need of an anticurl layer not only can reduce the belts unit manufacturing cost and increase belt yield and daily production through-put, but provide photoreceptor belts with extended mechanical functioning life and suppression of early onset of fatigue charge transport layer cracking problems. Although attempts have been made to overcome these problems, the solution of one problem often leads to the generation of additional problems.
In U.S. Pat. No. 5,089,369 to R. Yu, issued on Feb. 18, 1992, an electrophotographic imaging member having a supporting substrate and a charge generating layer, the supporting substrate material having a thermal contraction coefficient which is about the same as that of the charge generating layer. Substrate materials are disclosed that have a thermal contraction coefficient value between about 5.0xc3x9710xe2x88x925/xc2x0 C. and about 9.0xc3x9710xe2x88x925/xc2x0 C. for use in combination with a benzimidazole perylene charge generating layer.
U.S. Pat. No. 5,167,987 to R. Yu, issued on Dec. 1, 1992, discloses a process for fabricating an electrostatographic imaging member including providing a flexible substrate comprising a solid thermoplastic polymer, forming an imaging layer coating including a film forming polymer on the substrate, heating the coating and substrate, cooling the coating and substrate, and applying sufficient predetermined biaxial tensions to the substrate while the imaging layer coating and substrate are at a temperature greater than the Glass Transition Temperature (Tg) of the imaging layer coating to substantially compensate for all dimensional thermal contraction mismatches between the substrate and the imaging layer coating during cooling of the imaging layer coating and the substrate, removing application of the biaxial tensions to the substrate, and cooling the substrate whereby the final hardened and cooled imaging layer coating and substrate are free of internal stress and strain.
U.S. Pat. No. 4,983,481 to R. Yu, issued on Jan. 8, 1991, discloses an imaging member without an anti-curl backing layer is disclosed having improved resistance to curling. The imaging member comprises a flexible supporting substrate layer, an electrically conductive layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, the supporting substrate layer having a thermal contraction coefficient substantially identical to the thermal contraction coefficient of the charge transport. The supporting substrate may be a flexible biaxially oriented layer.
While the above mentioned flexible imaging members may be useful for their intended purpose of resolving specific problems and improving imaging members"" function, resolution of one problem has often been found to create new ones. For example, the selection of a supporting substrate, for example, polyether sulfone or MAKROFOL(copyright) having thermal contraction matching with that of the MAKROFOL(copyright) found in the coated charge transport layer to effect the suppression of electrophotographic imaging member curling, has been observed to be susceptible to attack and damage by solvents used in the charge transport layer coating solution, rendering the imaging member useless. Other substrate supports, having good thermal contraction matching properties such as TEDLAR or MELINAR, though yielding curl-free electrophotographic imaging members without anticurl back coating, have inherently low Glass Transition Temperatures (Tg), and were judged not suitable for imaging member fabrication. Application a biaxial tensioning stress onto imaging members maintained at an elevated temperature slightly above the Glass Transition Temperature (Tg) of the charge transport layer was found to be a cumbersome batch process, which is very costly to implement in imaging member production. There continues to be a need for improved methodology useful for fabricating imaging members, particularly specific substrate support material selection free of solvent attack and effects the elimination of anticurl backing layer, for multilayered electrophotographic imaging members fabrication to provide mechanically robust imaging member belts machines.
It is a feature of the present invention to provide an improved flexible multilayered electrostatographic imaging member belt involving selection of a substrate support material to effect the elimination of anticurl backing layer and render the imaging member belt flat.
Another feature of the present invention is to provide an improved methodology for fabricating flexible electrostatographic imaging member webstocks that minimize solvent emission to the environment.
The present invention in embodiments provides an improved multilayered flexible electrostatographic imaging member webstock production method that cuts costs, reduces yield lost and increases daily imaging member webstock production through-put;
an improved flexible multilayered electrostatographic imaging belt having a reduced seam splashing size to ease cleaning blade mechanical sliding action as well as minimize blade wear, as well as improving the imaging member belt motion quality during dynamic belt machine function;
an improved multilayered flexible electrophotographic imaging member belt having a charge transport layer that is free of internal stress and strain;
an improved multilayered flexible electrophotographic imaging member belt with improved resistance to premature onset of dynamic fatigue bending induced charge transport layer cracking as well as suppressing the development seam cracking and delamination when imaging member belt cyclic flexing over various belt support module rollers under machine imaging function conditions;
an electrophotographic imaging member comprising a flexible substrate support layer selected for the present invention application then coated over with an electrically conductive substrate surface layer, a hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer having a thermal contraction coefficient value substantially matched to that of the substrate support layer. To yield the desired imaging member flatness without the requirement of an anticurl backing layer, the substrate support layer and the charge transport layer have a thermal contraction coefficient difference of from about xe2x88x922xc3x9710xe2x88x925/xc2x0 C. to about +2xc3x9710xe2x88x925/xc2x0 C.; and in embodiments, a difference in thermal contraction coefficient of from about xe2x88x921xc3x9710xe2x88x925/xc2x0 C. to about +1xc3x9710xe2x88x925/xc2x0 C. In a specific embodiment, the difference in the thermal contraction coefficient between the substrate support and charge transport layer is from about xe2x88x920.5xc3x9710xe2x88x925/xc2x0 C. and about +0.5xc3x9710xe2x88x925/xc2x0 C. Furthermore, the selected substrate support should also have a Glass Transition Temperature (Tg) of at least 100xc2x0 C., wherein the substrate support is not susceptible to attack by the solvent used in the charge transport layer coating solution, and can also conveniently be welded into an overlapped seamed flexible imaging member belt by an ultrasonic seam welding process. One substrate support is a modified thermoplastic polyimide represented by the following formulas: 
wherein,
m, n, and q represent the degree of polymerization for example numbered from about 10 to about 300, or from about 50 to about 125 and
x, y, represent the number of segments and z, the number of repeating units are integers, for example, x and y are from about 2 to about 10, or from about 3 to about 7. Whereas z is from about 1 to about 10, or from about 3 to about 7.
The discussions hereinafter relate to fabricating flexible electrophotographic imaging member belts (photoreceptor belts) and are equally applicable to fabricating electrographic imaging members (e.g., ionographic belts).
Flexible electrophotographic imaging member belts generally comprise a flexible supporting substrate having an electrically conductive surface layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an anticurl backing layer, an optional ground strip layer and an optional overcoating layer. The flexible substrate support layer which in embodiments may be transparent and have a thickness of about 25 micrometers to about 200 micrometers. A thickness of from about 50 micrometers to about 125 micrometers gives optimum light transmission and a rigid substrate support layer. The conductive surface layer coated over the flexible substrate support may comprise any electrically conductive material such as, for example, aluminum, titanium, nickel, chromium, copper, brass, stainless steel, silver, carbon black, graphite, and the like. The electrically conductive surface layer coated above the flexible substrate support layer may vary in thickness over a substantially wide range depending on the desired usage of the electrophotographic imaging member. However, in embodiments, the thickness of the conductive surface layer may be from about 20 Angstroms to about 750 Angstroms. It is, nonetheless, desirable that the conductive surface layer coated over the flexible substrate support layer have a thickness from about 50 Angstroms to about 120 Angstroms in thickness to provide sufficient light energy transmission of at least 20% transmittancy to allow effective imaging member belt back erase.